Fix: Handle null values in PlainFixedLenByteArrayEncoder gracefully

[singh1203]

Fixes: #71

Rationale for this change

Ensures PlainFixedLenByteArrayEncoder handles null values gracefully in non-nullable fields by writing zero-filled bytes instead of panicking, maintaining consistency with DictFixedLenByteArrayEncoder.

What changes are included in this PR?

• Replaces panics on null values with writing zero-filled bytes to maintain byte alignment.
• Ensures encoding consistency between dictionary and plain encoding.
• Prevents unexpected failures when handling schema inconsistencies.

Are these changes tested?

Yes:

• Passed existing test suite.
• Verified with local Arrow-Go modifications.

Are there any user-facing changes?

Not Sure (Need possible guidance if required)

[zeroshade]

Can we add a new test that would fail without this change?

[singh1203]

Can we add a new test that would fail without this change?

Sure @zeroshade I'm on it.

[singh1203]

Thank you. @zeroshade